This is an incredible game, I've almost got 300 hours into it and I'm still looking forward to firing it up nearly every day.
There's a lot of reasons to buy this game:
* if you love space, this game will get you there, first hand, and let you have your hand at designing space crafts (and planes), flying them, and of course, crashing them in big glorious explosions
* you will learn a lot about space, physics, orbital mechanics, aerodynamics, etc... this really happens, I have two friends that are rocket scientists (one does ion drive research, the other designs micro-satellites, I work at a large university), and after playing this game for a few months, I could easily talk to them about their work, where they drop terms like ISP, TWR, retrograde, normals, etc, terms that I now fully understand
* the game is very approachable, you can play in "career mode" which walks you though the beginning steps of putting a rocket into space, to orbiting, to visiting the Mun and beyond. If you need more help, the YouTube scene is quite impressive, with people like Scott Manley producing really professional (and charming) video tutorial series. I am not a lover of simulation-type games, I play mostly FPS's and RPG's, but I found this game quite easy to pick and up get into.
* while approachable, this game is certainly not easy, for example, landing and returning from Eve (the Venus-like planet in the Kerbin system) is one of the hardest tasks in the game, and while most of the game is fairly non-stressful in its nature, landing spaceships is a tense experience, especially when Jeb's life is on the line! For those that find it difficult to master some of the more difficult tasks, there are impressive control mods like MechJeb2 where you essentially point at a spot on a planet and press the "Land There" button, MechJeb does the rest.
* the game has a unique charm to it, in the game you are progressing through the space race for the people of the Kerbin panet, you'll get to know and love the kerbalnauts that will fly your craft, especially Jebediah Kerman, pilot extraordinaire and fearless hero to the Kerbin people
* the community for this game, is active, organized, and quite friendly, mostly found on Reddit and the Squad forums, you can get any question answered, or read mission reports from someone's latest mission, or get news on upcoming updates for the game. The only time you see people getting belligerent on these forums is when some is misbehaving toward others
Squad is actively developing this game, each release (every new months) bringing a lot of new features and parts to build with, and just this week the update to KSP brings many new rocket parts built in collaboration with NASA plus a mission to save Kerbin from killer asteroids!
* the mod scene for this game is extremely impressive, from new real-life inspired rocket parts (so you can build your own Saturn V), to impressive new systems that enable for example planetary mining (so you can get more fuel on route to your target), to just plain fun mods like the nuclear pulse rocket, which explodes varied-sized nuclear bombs behind a large plate to produce thrust (scientists actually design this beast in the 50's)
* the endgame is rich and deep: once you've completed career mode, many KSP players switch to the realism mods, which give you the real solar system, truly deadly reentries, realistic communication constraints (that require building satellite networks), more realistic aerodynamics modeling (e.g., Ferram's amazing FAR mod), real life-support constraints, etc. that make for a significantly more challenging and rewarding experience; if that doesn't trip your trigger, the forums have organized challenges with leaderboards, to see who can, for example, get to the Mun with the least number of parts, or eject the most Kerbals out of the solar system, all great fun with the competitors meticulously documenting their missions
If these things sound interesting to you, get this game! It was the best $25 bucks I ever spent!